This invention relates to an improved self supporting optical diffraction grid for short wavelength electromagnetic radiation, such as extreme ultraviolet and x-ray radiation.
Such diffraction grids are used, for example, in the investigation of extraterrestrial radiation sources (extreme ultraviolet and x-ray sources). In many cases, such diffraction grids are installed in suitable optical instruments which are placed into earth orbit outside the earth's atmosphere by rocket launch vehicles. Such diffraction grids must therefore exhibit the necessary optical properties and in addition must exhibit mechanical properties that are compatible with the heavy mechanical loads on the diffraction grids bearing the launch phase.
In order to function properly in terms of optical characteristics, such diffraction grids must have extremely fine grain grid structures in which the grid constants often are less than one micron. Stresses associated with sound waves encountered during the launch phase present particularly serious threats to the integrity of such grids. These diffraction grids include grid foils which, when excited by periodic air pressure fluctuations, are exposed to a continuous bending stress. Generally, the smallest bending radii and therefore the most severe material fatigue occur in a narrow transition zone between the relatively flexible grid foil and a relatively rigid surrounding frame.
In the article entitled "XUV Filters-Out Of This World And Back Again" published in Optical Spectra, June, 1976, at pages 37-40, methods for ruggedizing homogeneous filter foils are disclosed. The method disclosed at page 38 involves the application of a layer of an inorganic material which dries in the manner of a lacquer to the edges of the foil to improve its mechanical stability. This method, however, is not directly applicable to diffraction grid foils of the type described above, because in consequence of the capillary action within the grid foil, the width of the solidification zone of the organic material cannot readily be maintained at a desired predetermined width, and often no wedge-shaped film of organic material develops. In addition, the capillary action mentioned above can often result in undesirably wide border zones which can reduce the usable aperture of the grid foil to an unacceptable extent.